Super luminescent diodes (SLDs) having structures similar to those of semiconductor laser elements have a broad light emission spectrum close to that of light emitting diodes (LEDs) and can emit light at narrow emission angles and with high light intensities equivalent to the semiconductor laser elements. Also, super luminescent diodes have been applied in the field of interferometers such as optical gyroscopes in the related art, but have recently been receiving attention in an application to display devices as light sources for forming images having little interference noise (speckle noise) because of their low coherence.
For example, a waveguide is formed to be bent in the middle thereof so that reflection from an end surface is remarkably suppressed, or oscillation of a laser mode is suppressed to realize a low coherent light source using a subminiature short resonator in super luminescent diodes disclosed in, for example, JP H2-310975A. In other words, an amplified state is acquired by passing light through a waveguide only in one direction thereof rather than resonance occurring because of reciprocation of light between two end surface mirrors like semiconductor laser elements. A generation source of light is spontaneously emitted light generated from a portion of an active layer, and the spontaneously emitted light having a wide light emission spectrum width is directly amplified and is emitted to the outside.